The hypothesis to be tested here is that molecular polymorphisms in the pfcrt (Plasmodium falciparum {PJ] chloroquine resistance transporter) and pfmdrl (Pf multi-drug resistance) genes associated with chloroquine resistant (CQR) Pf alter the physiology of the parasite's digestive vacuole (DV) where chloroquine' s (CQ) anti-parasite activity is believed to occur. Malaria is the leading cause of morbidity and mortality in PNG, and as drug treatment is the only effective means of control for clinical malaria, an improved understanding of mechanisms by which drug resistance evolves, will have an immediate impact on PNG health care policy. Our preliminary studies showed that PNG Pf isolates carry pfcrt alleles associated with either CQ sensitivity (CQS, CVMNK) or resistance (CQR, SVMNT) observed previously in Brazilian Pf strains and not the CQR associated CVIET allele observed in more proximal Southeast Asian strains. Further, the CQR phenotype observed in PNG and South American PfSVMNT strains was marked by significantly reduced sensitivity to in in vitro verapamil (VPL) reversal than was observed for CVIET strains. We hypothesize that these pfcrt sequence differences are responsible for CQR characterized by reduced VPL reversibility. As VPL reversibility is considered to be an integral component of PfCQR, clarifying the molecular basis of this phenotype may reveal new insight regarding parasite physiology and promote new development strategies for future anti-malarial drugs. The specific aims are to: 1) Conduct antimalarial treatment therapeutic efficacy trials and from parasites isolated from study volunteers, 2) Correlate in vitro CQ susceptibility phenotypes for PNG Pf with pftrt and pfmdr1 sequence polymorphisms, and 3) Apply newly developed allele replacement strategies to determine how pfcrt and pfmdr1 sequence polymorphisms influence CQ IC50, CQR/VPL reversal and regulation of parasite DV physiology.